Owing to the rising world demand for electric energy and the limited reserves of coal, oil and gas, which in addition liberate the greenhouse gas CO2 on combustion, the generation of electric power from sunlight has attracted increased interest in recent years.
A photovoltaic cell is a component in which light is converted directly into electric energy. In addition to two electrodes, it comprises at least one light-absorbing layer and a charge transport layer. If the light is sunlight, one speaks of a solar cell. Although the photovoltaic effect, i.e. the generation of an electric potential across a pn junction on irradiation with light, was observed by Becquerel as early as 1893, the first functional solar cells were only able to be produced with the development of semiconductor technology in the 1940s and 1950s. Since the voltage of a single solar cell is generally small (typically  less than 1V), cells are usually connected in series to form modules.
The solar cells used nowadays usually comprise a semiconductor material, in most cases silicon, as light-absorbing layer. However, the silicon used in this application has to meet very high demands in respect of purity and quality of processing. As a result, solar cells are at present not competitive for many applications for cost reasons.
In contrast, a dye-sensitized solar cell developed in the 1960s by H. Tributsch uses a material having a very large band gap, e.g. titanium dioxide, as semiconductor. Such a semiconductor absorbs little sunlight, for which reason a dye (chromophore) is applied to the semiconductor.
If a dye molecule absorbs a photon, this causes excitation of an electron into the lowest unoccupied molecular orbital. From this, it can then be injected into the conduction band of the semiconductor. The semiconductor thus serves predominantly for the transport of electrons. For this purpose, no particular purity and perfection of the material is necessary. The semiconductor layer can, for example, simply be painted from a powder suspension onto conductive glass. EP-A 0 333 641 describes a photoelectrochemical cell which comprises a nanoporous metal oxide semiconductor, i.e. a semiconductor having an extremely roughened surface and thus an increased surface area. In this cell, charge transport between semiconductor/chromophore layer and counterelectrode occurs through an electrolyte solution. Although good results are achieved using such cells, the property profile of such a device is still capable of considerable improvement. The electron is returned to the dye by diffusion of ions. As a result, the only suitable redox systems are those which are small enough to penetrate into the pores of the nanocrystalline semiconductor layer. In the best redox system up to the present time, viz. Ixe2x88x92/I3xe2x88x92, about 40% of the energy which is theoretically available is lost as heat because of poor matching of the energy levels between dye and redox system, and the efficiency of energy conversion is limited to about 10% in direct sunlight. A further disadvantage of this redox system is that the chemically aggressive triiodide places extreme demands on sealing materials and also on insulators and conductors which are necessary for connecting the cells in series in modules. This disadvantage means that it is very difficult to produce stable cells and modules and that the production costs increase because of the complicated sealing technique.
It has now surprisingly been found that the above described disadvantages can be reduced if the electrolyte comprising the Ixe2x88x92/I3xe2x88x92 redox system in the above-described cell is replaced by an electrolyte in which a hole conductor compound assumes the role of the redox system.
The invention accordingly provides a photovoltaic cell comprising an electrolyte which comprises a hole conductor compound as redox system. The redox system preferably consists of one or more hole conductor compounds. The use of a hole conductor compound in place of Ixe2x88x92/I3xe2x88x92 enables the energy levels of redox system and dye to be matched more closely so as to make possible better utilization of the absorbed energy so that the efficiency for sunlight can be improved. In addition, replacement of the aggressive triodide by hole conductor compounds makes it possible to use less expensive sealing layers and insulators and conductors and also makes possible simpler production processes for cells and modules.